1. Technical Field
The invention relates to a piston for an internal combustion engine.
2. Related Art
In internal combustion engines a fuel-air mixture is periodically ignited and burned at the end of a compression stroke and at the beginning of an expansion stroke. In reciprocating piston engines, this happens in a combustion chamber which is delimited from a crankcase by a piston which moves up and down in a cylinder. The piston is connected to a crank shaft via a connecting rod. A piston pin serves in turn to connect the piston to the connecting rod, the piston pin being accommodated in a pin boss formed in the piston.
In recent years there has been a growing trend with engines towards direct fuel injection in conjunction with turbocharging. This development entails new challenges for the development of pistons, in particular in view of the steadily increasing ignition pressures together with the ever later center of gravity position of the combustion, which manifest as high lateral loads for the piston skirt. Owing to the increased loads, conventional piston concepts are increasingly unsuitable for fulfilling the requirements, particularly in view of the required durability.
In order to cope with the increased loads, DE 10 2009 032 379 A1 discloses a piston having pressure-side box walls which run in a straight line and at an inclination such that the distance of the box walls in the region of the pin boss is greater than in the region of the pressure-side skirt wall, This is supposed to improve the skirt/box wall connection.
EP 0 913 566 A2 describes a cooled piston for internal combustion engines, in which a cooling duct is formed inside the piston, which is characterized by an entry point that lies outside of the piston skirt. Improved cooling is achieved in this way.
DE 10 2008 002 536 A1 describes a piston characterized on the pressure side by connecting walls that are largely straight and by curved walls on the counter pressure side. This reduces the generation of noise and contributes to greater resilience.
A piston is revealed by DE 40 19 968 C2, the so-called skirt portion of which having on the outer side an elliptical form at least in some regions.
Pistons having skirt walls and box walls have also become known under the designations “slipper piston” and “slipper skirt”, which are supposed to ensure high resilience with a low weight.
With pistons subject to high loads, there is a tendency for box wall cracks to appear in the region of the connection of the box wall to the piston skirt. These cracks are caused by a bending moment which occurs during the deformation of the piston skirt under the ignition pressure and acts in the region of the intersection between the piston skirt and the box wall. Conventional methods, in particular trying a smaller or larger intersection radius between the box wall and the side wall, have not been able to solve this problem in a satisfactory manner.